The invention is concerned with a miniature x-ray tube on a probe, for use in narrow lumens such as human blood vessels, or with an applicator in natural or surgical cavities, for delivering an x-ray dosage. More specifically the invention is directed at such an x-ray tube including a micro cathode formed by MEMS techniques.
A miniature x-ray tube is disclosed in Xoft Microtube U.S. Pat. No. 6,319,188, and that disclosure is incorporated herein by reference. As explained in that patent, it is sometimes desirable to irradiate the interior walls of blood vessels following balloon angioplasty or the implantation of a stent. Such radiation, which has in the past been administered by radioactive isotopes at the end of probes, has been found useful in prevention and treatment of restenosis. Other cavities or passages, both surgical and natural, can also be beneficially treated by x-ray, provided the source is relatively small.
Much of the previous effort for miniature x-ray tubes has focused on field emission cathodes. Field emission cathodes (gated and non-gated) have a number of disadvantages including extreme vacuum requirements; very short lifetimes in high voltage x-ray tubes due to ion milling of the cathode; lack of robustness from tube arcs, which are common in high voltage x-ray tubes; anode to cathode spacing requirements; and limited x-ray tube output. The high vacuum requirement is especially severe in gated types since the surface to volume ratio for these small sources is so large and space for getter material is limited. Thermionic cathodes, on the other hand, are tolerant of poor vacuum conditions and tube arcs.
In fabricating a functional, reliable and efficient miniature thermionic cathode x-ray tube for such purposes, the size of the x-ray tube at the end of the probe, and the functional considerations for the device both in operation and for insertion, dictate a number of limitations. The tube itself must be very small in diameter: for lumens, on the order of about 1.00 to 1.25 millimeter in external diameter, and less than 9 mm in length, preferably 5-6 mm; for other cavities, generally about 2-3 mm in diameter. The probe cable for lumens is required to be very flexible, capable of turning around a radius on the order of about one centimeter, which generally dictates that the probe diameter and length be small. High voltage must be carried in the probe cable over two conductors connected to a cathode and an anode of the x-ray tube, with sufficient insulation and spacing to prevent dielectric breakdown. The thermionic cathode is required to operate on very small current, or else the conductors within the probe cable serving the cathode heating circuit will need a large cross section, and this can compromise the spacing between the high voltage conductors and ground, thus compromising dielectric integrity or necessitating a larger diameter in the probe. A diameter larger than about 1 mm to 1.25 mm, or reduced cable flexibility, will compromise the usefulness of the x-ray probe for treatment of restenosis or other small lumens or vessel indications.
All of these concerns make difficult the design and production of an effective and efficient miniature x-ray probe device using an extruded metal wire filament as a thermionic cathode. This and other issues are addressed by the invention described below.